The present invention relates to a displacement control valve incorporated in variable displacement compressors that are used in vehicle air conditioners.
A typical variable displacement compressor has a supply passage for connecting a discharge chamber with a crank chamber and a displacement control valve located in the supply passage. The displacement control valve controls opening amount of the supply passage for adjusting the amount of highly pressurized refrigerant gas that is supplied to the crank chamber from the discharge chamber. The pressure in the crank chamber is changed, accordingly. This alters the difference between the pressure in the crank chamber and the pressure in cylinder bores. Changes in the pressure difference adjust the inclination of a swash plate of the compressor and ultimately change the displacement of the compressor.
A typical displacement control valve has a housing and a valve chamber defined therein. The valve chamber is connected to a supply passage of the compressor by a valve hole. The valve chamber and the valve hole constitute a part of the supply passage. A valve body is accommodated in the valve chamber for opening and closing the valve hole. A solenoid is attached to the housing. The solenoid includes a fixed steel core, a plunger and a coil. The plunger moves closer to and away from the fixed core. The plunger is operably coupled to the valve body by a rod. Feeding current to the coil of the solenoid generates magnetic attractive force between the fixed core and the plunger. The magnitude of the generated force corresponds to the magnitude of the fed current. Therefore, the opening area between the valve hole and the valve body is controlled by changing the magnitude of current fed to the coil. Accordingly, the amount of refrigerant gas passing through the supply passage is changed.
The rod, which couples the valve body with the plunger, extends through and is supported by a guide hole. The plunger is movably housed in a plunger chamber defined in the solenoid. Therefore, refrigerant gas in the valve chamber may leak to the plunger chamber through minute clearance between the rod and the guide hole. Misted lubricant oil contained in the refrigerant gas is also introduced to the plunger chamber.
Some compressors have a passage that communicates the plunger chamber with a crank chamber or with a discharge chamber for positively introducing refrigerant gas to the plunger chamber. This construction equalizes pressures acting on both sides of the valve body and thus stabilizes the valve body operation without being affected by refrigerant gas pressure. This construction also increases the amount of lubricant oil that is introduced to the plunger chamber.
The lubricant oil in the plunger chamber is spread on the wall of the chamber and on the surface of the plunger. The oil thus causes the plunger to adhere to the wall of the plunger chamber and hinders the movement of the valve body. Specifically, lubricant oil may cause the end of the plunger to adhere to the inner wall of the plunger chamber when the solenoid is not excited. If excited in this state, the solenoid does not quickly attract the plunger. In this manner, lubricant oil in the plunger chamber hinders rapid movement of the valve body.
Especially, in a displacement control valve that controls the opening between a valve body and a valve hole by changing the magnitude of a solenoid coil current, the movement of the plunger must follow the subtle changes of the coil current. If a plunger adheres to a surface due to lubricant oil, the plunger fails to accurately follow subtle changes in the coil current. The opening of the valve hole is thus not accurately controlled.